Reinforcement with particles

By the term particulate composites we are referring to composites reinforced with particles having dimensions of the same order of magnitude. Particulate composites are produced from a polymeric matrix, into which a suitable metal powder has been dispersed, and exhibit highly improved mechanical properties, better electrical and thermal conductivity than either phase, lower thermal expansivity, and improved dimensional stability and behaviour at elevated temperatures. 

Reinforcement with particles such as chalk, quartz, mica and glass spheres, is frequently carried out with thermoplastics and thermosets to obtain a higher stiffness (and sometimes a higher strength). There is a gradual transition from high-quality to... 

Table 13.1 % Water absorbance by neat UPE and its composites reinforced with particle and short form of raw, mercerized, silanated, benzoylated, Grewia optiva-g-poly (AN) and Grewia optivfl-g-poly-(AAc) fibers.

Bio-nanocomposites represent a stimulating route for creating new and innovative materials, where a large variety of superior properties have been realised and the opportunity to identify further advancements in the property behaviour is immense. These materials consist of a biopolymer matrix reinforced with particles having at least one dimension in the nanometer range i.e. 1-100 nm. Remarkable improvements have been reported for the mechanical, thermal and barrier properties of bio-nanocomposites in contrast to the base biopolymers [6]. This in turn makes this new class of materials very favourable for numerous end use apphcations. 

For examining the surface morphology of polymeric UPE resin and UPE-based polymer composites reinforced with particle form of raw and mercerized C. indica fibers, scanning electron microscopy is an excellent technique. It has been observed... 

Polymer nanocomposites consist of a polymer matrix with embedded filler particles with at least one dimension at the nanometre level, (i.e. 1-100 nm), much smaller than for the conventional polymer composites described above. The inclusion of nanoparticles can effect significant improvements in mechanical properties such as modulus, yield stress and fracture toughness for filler levels as low as a few per cent by weight. This is much lower than in conventional polymer composites, as illustrated in Figure 9.7, where the effect of talc reinforcement and clay nanoparticle reinforcement in a polypropylene matrix are compared. Talc filler is regarded as a conventional reinforcement, with particle diameters in the range 1-10 qm and thickness around 20 times less, whereas the clay particles are of length around 100 nm and thickness as low as 1 nm. Clay occurs in the form of platelets and has been... 

Other eomposite material composed with a glass-ceramic is Ceravital reinforced with titanium particles [119] and AAV glass-ceramic reinforced with particles of zireonia partially stabilized [120]. 

FluorosiHcones (FVMQ) have exceUent low temperature flexibUity properties coupled with good oil, fuel, and solvent resistance and exceUent aging properties. The materials are compounded and reinforced with fine particle fiUers, especiaUy sUica. The materials are mixed and processed on especiaUy clean equipment and are peroxide-cured. 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode of failure. 

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

Ceramic matrix composites are candidate materials for high temperature stmctural appHcations. Ceramic matrices with properties of high strength, hardness, and thermal and chemical stabiUty coupled with low density are reinforced with ceramic second phases that impart the high toughness and damage tolerance which is required of such stmctural materials. The varieties of reinforcements include particles, platelets, whiskers and continuous fibers. Placement of reinforcements within the matrix determines the isotropy of the composite properties. 

When employed in elastomeric systems it is commonly observed that the finer the particle size the higher the values of such properties as tensile strength, modulus and hardness. Coarser particles will tend to give compounds less strong than compounds with the filler absent, but if the particle size is sufficiently fine there is an enhancement in the above-mentioned properties (at least up to an optimum loading of filler) and the phenomenon is known as reinforcement. The particle shape also has an influence for example, the somewhat plate-like china... 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

Calcium silicate produced by precipitation is a fine powder with particle sizes down to 1 uni. It is a reinforcing filler with a reactivity greater than aluminium silicate. It requires the use of additional accelerator as it slightly retards the vulcanisation reaction. 

Fig. 9.4.15 High-resolution transmission electron micrograph (HRTEM) of Mg nanoparticles on a carbon-reinforced microgrid. Particles were prepared by a matrix isolation method with He gas. Each particle has a clear crystal habit, presumably hexagonal in shape. (From Ref. 20.)...

Particulate Composites. Particulate composites encompass a wide range of materials, from cement reinforced with rock aggregates (concrete) to mixtures of ceramic particles in metals, called cermets. In all cases, however, the particulate composite consists of a reinforcement that has similar dimensions in all directions (roughly spherical), and all phases in the composite bear a proportion of an applied load. The percentage of particulates in this class of composites range from a few percent to 70%. 

Figure 8.4 Stress-strain isotherms for PDMS networks reinforced with in situ generated titania particles.39 Each curve is labeled with the wt % of filler introduced, and filled circles locate results used to test for reversibility.

The attraction of reinforcing ceramic matrices with particles or whiskers is that, with appropriate microstructural design and property tailoring, materials with property combinations not possible in monolithic ceramics can be obtained. In addition, the materials remain effectively isotropic and can be manufactured by well-established techniques already in use for the manufacture of monolithic ceramics (Hansson and Warren, 2000). 

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